There is known a device in a vehicle including a torque converter having a lockup clutch between an engine and a transmission. In this device, when the lockup clutch is shifted from a disengagement region to an engagement region, a ramp pressure is started to be increased by a ramp control. Then, a lockup capacity is increased by the pressure increase by the ramp control. When it is shifted in a region in which a pump rotation speed (=an engine speed) of the torque converter corresponds to a turbine rotation speed, the lockup clutch is engaged (cf. a patent document 1 and so on).
However, in the conventional device, an increase gradient of the ramp pressure difference is maintained to a constant value from the start of the ramp control to the end of the ramp control, irrespective of a turbine rotation speed variation amount of the torque converter. Accordingly, when the engine speed is suddenly decreased in the end region of the ramp control in which the turbine rotation speed variation amount is small, the engine rotation speed may be varied by the sudden engagement of the lockup clutch, so that the engagement shock may be generated.
For example, a case where the ramp control to engage the lockup clutch is started at the reacceleration by the accelerator depression operation from the accelerator foot release coast deceleration in the disengagement state of the lockup clutch is explained. In the turbine rotation speed during the ramp control, the turbine rotation speed is increased by the downshift of the transmission in the start region of the ramp control. However, the turbine rotation speed may be hardly varied in the end region of the ramp control by the upshift with respect to the increase of the vehicle speed. On the other hand, in the engine speed during the ramp control, the engine speed is increased in the start region of the ramp control since there is no lockup capacity. However, the engine speed is suddenly decreased in the end region of the ramp control from the increased engine speed toward the turbine rotation speed. Accordingly, at the clutch engagement in which the engine speed is converged to the turbine rotation speed, an angle formed by the engine rotation speed characteristic and the turbine rotation speed characteristic becomes large.